The present application relates to an optical device and a virtual image display for guiding display image light as a virtual image to viewer's pupils through the use of a reflection type volume hologram grating.
International Publication No. 2005/093493 pamphlet proposes a device allowing a viewer to observe a two-dimensional image displayed on an image display element as an enlarged virtual image by a virtual image optical system using a reflection type volume hologram grating. The device is a display applicable as, for example, an HMD (Head Mounted Display). FIG. 18 illustrates a configuration example of a virtual image display 80 proposed by International Publication No. 2005/093493 pamphlet.
The virtual image display 80 includes an image display element 81 displaying an image, and a virtual image optical system receiving display light displayed on the image display element 81 and then guiding the display light to a viewer's pupil 16. The image display element 81 is, for example, an organic EL (Electro Luminescence) display, an inorganic EL display, a liquid crystal display (LCD) or the like. The virtual image optical system includes a collimating optical system 82 and a light guide plate 83 including a hologram layer 84 arranged therein. The collimating optical system 82 is an optical system receiving light beams emitted from pixels of the image display element 81, and then converting the light beams into a plurality of parallel light beams with different view angles. The plurality of parallel light beams with different view angles emitted from the collimating optical system 82 enters the light guide plate 83.
FIG. 18 illustrates, as a representative of the parallel light beams, only a parallel light beam L10 with a central view angle which is emitted from a pixel in a central part of the image display element 81, and then converted into a light beam with a zero view angle (vertical to an incident surface of the light guide plate 83) by the collimating optical system 82 to enter the light guide plate 83.
The light guide plate 83 has a configuration in which the hologram layer 84 is sandwiched between transparent substrates 83A and 83B. The light guide plate 83 is a light guide plate in the shape of a thin parallel plate including, as main surfaces, an optical surface 83a and an optical surface 83b facing the optical surface 83a. The optical surface 83a has a light inlet 83a1 at one end thereof to receive the parallel light beams with different view angles emitted from the collimating optical system 82. The optical surface 83a has a light outlet 83a2 at the other end thereof to emit light. Protective sheets 85 and 86 for protecting the optical surfaces 83a and 83b are arranged on the optical surfaces 83a and 83b of the light guide plate 83, respectively. Moreover, a light-shielding plate 87 is arranged on the protective sheet 86 arranged on the optical surface 83b in the same position as that of the light inlet 83a1 of the light guide plate 83 to prevent a decline in light use efficiency caused by leakage of an enlarged image displayed on the image display element 81 and enlarged by the collimating optical system 81 to outside of the light guide plate 83.
In the hologram layer 84, a first reflection type volume hologram grating 84a, hereinafter described as a first grating 84a, is formed in a position corresponding to the light inlet 83a1, and a second reflection type volume hologram grating 84c, hereinafter described as a second grating 84c, is formed in a position corresponding to the light outlet 83a2. A section where the first and second gratings 84a and 84c are not formed of the hologram layer 84 is a non-interference-fringe-recording region 84b where interference fringes are not recorded. In the first grating 84a, interference fringes are recorded with uniform pitches on a hologram surface. Moreover, in the second grating 84c, interference fringes having different diffraction efficiency depending on their positions are recorded. The second grating 84c has lower diffraction efficiency in a position near the light inlet 83a1 and higher diffraction efficiency in a position far from the light inlet 83a1 so that light is allowed to be diffracted and reflected a plurality of times.
The parallel light beams with different view angles entering from the light inlet 83a1 of the light guide plate 83 enter the above-described first grating 84a, and each of the parallel light beams is diffracted and reflected as it is. The diffracted and reflected parallel light beams travel while being totally reflected between the optical surfaces 83a and 83b of the light guide plate 83 to enter the above-described second grating 84c. The light guide plate 83 is designed to have a sufficient length in a longitudinal direction and a thin thickness between the optical surface 83a and the optical surface 83b so as to have such an optical path length that numbers of times of the total reflection of the parallel light beams with different view angles, while traveling inside the light guide plate 83 until the parallel light beams arrive at the second reflection grating 84c, depend on their view angles.
More specifically, among the parallel light beams entering the light guide plate 83, a parallel light beam entering the light guide plate 83 while being slanted toward the second grating 84c, that is, a parallel light beam with a large incident angle is reflected a smaller number of times than a parallel light beam entering the light guide plate 83 while being hardly slanted toward the second grating 84c, that is, a parallel light beam with a small incident angle, because the parallel light beams entering the light guide plate 83 have different view angles from one another. In other words, the incident angles of the parallel light beams to the first grating 84a are different from one another, so the parallel light beams are diffracted and reflected at different diffraction angles, thereby leading to total reflection at different angles. Therefore, when the light guide plate 83 has a lower profile and maintains a sufficient length in the longitudinal direction, the numbers of times of the total reflection of the parallel light beams are pronouncedly different from one another.
The parallel light beams with different view angles which enter the second grating 84c are diffracted and reflected thereby to deviate from conditions of total reflection, and then the parallel light beams are emitted from the light outlet 83a2 of the light guide plate 83 to enter the viewer's pupil 16.
In the virtual image display 80, when the diffraction efficiency of the second grating 84a is changed depending on position, a pupil diameter, that is, the virtual image viewable range of the viewer is expanded. More specifically, for example, when the diffraction efficiency of the second grating 84c is 40% in a position 84c1 near the light inlet 83a1 and 70% in a position 84c2 far from the light inlet 83a1, 40% of the parallel light beams entering the second grating 84c for the first time is diffracted and reflected in the position 84c1, and 60% of the parallel light beams passes through. The parallel light beams having passing through are totally reflected inside the light guide plate 83, and enter the position 84c2 of the second grating 84c. 
The diffraction efficiency in the position 84c2 is 70%, so 60% of the parallel light beams passes through in the first entry into the second grating 84c, so 42% (0.6×0.7=0.42) of the parallel light beams is diffracted and reflected in the position 84c2. Thus, when the diffraction efficiency is appropriately changed depending on the position of the second grating 84c, the light intensity balance of light emitted from the light outlet 83a2 may be kept. Therefore, when a region in which the interference fringes are recorded of the second grating 84c is increased in the hologram layer 84, the virtual image viewable range is easily expanded.